Nondietary factors, such as age, race, specific genes, and the presence of hypertension, diabetes, or kidney disease, also affect the blood pressure response to changes in dietary sodium intake. Specifically, older-age persons, African-Americans, hypertensive individuals, and persons with diabetes or chronic kidney disease tend to be more salt sensitive than their counterparts. There is also demonstrated heterogeneity in the extent of cardiovascular disease risk reduction from a given reduction in blood pressure.

In the UL model (see Chapter 3), when there is concern that adverse effects may occur at levels of intake lower than the LOAEL or NOAEL, an uncertainty factor (UF) is used to adjust downward the LOAEL or NOAEL in order to derive the UL. The UL is defined as the highest level of intake consumed on a chronic basis at which no increased risk of serious adverse effects will occur. As indicated in Chapter 3, the UF is set at 1.0 when there is convincing evidence that the identified adverse effects do not occur at the observed NOAEL, but do occur at higher levels. The UF is set at greater than 1.0 when there is less convincing evidence that a true NOAEL has been demonstrated—there remains the possibility that adverse effects may occur at intakes below the NOAEL, even though they have not been documented. The UF is also greater than 1.0 when data demonstrating a NOAEL are unavailable, but data indicating a LOAEL are available.

For sodium, the UF could be set at greater than 1.0, because there are large numbers of persons who would achieve an even lower blood pressure by reducing their sodium intake from the LOAEL to lower levels. However, the actual NOAEL for these individuals is unknown. Choosing a level of sodium intake at which no one would experience a rise in blood pressure would be difficult because there is heterogeneity in both the extent of blood pressure reduction that would be achieved and in the extent of cardiovascular disease risk reduction. Also, consuming a diet with sodium intake level at the NOAEL may well result in a diet inadequate in other essential nutrients, particularly for those with lower levels of energy expenditure. Lastly, a UF of approximately 1.6 or higher would lead to a UL below the AI. In view of these considerations, the UF for sodium is set at 1.0.

Derivation of a UL. The LOAEL of 2.3 g (100 mmol)/day was divided by the UF of 1.0 to derive a UL of 2.3 g (40 mmol)/day for total sodium intake.